The invention relates generally to purification of feed gas used for the manufacture of ammonia, and more particularly to improvements in processing of feed gas from which hydrogen rich ammonia synthesis gas, and waste gas, are derived. The invention particularly concerns treatment of the waste gas to derive useful gas streams, one of which is nitrogen rich, and another is methane rich.
In prior purification processes, waste gas is separated from the synthesis gas, and it contains excess nitrogen from the feed gas, a small amount of hydrogen, all of the incoming methane, and about 60% of the incoming Argon. Such waste gas is typically utilized as fuel in a primary reformer. Improvements in treatment of the waste gas are needed, for enhanced overall process efficiency.
It is a major object of the invention to provide improvements in treatment of such waste gas, as will be seen. Basically, the improved process of the invention derives two product streams from the waste gas, one of which is nitrogen rich, and the other of which is methane rich, with a higher heating value than in processes employed so far, more suitable for use as a fuel, with less nitrogen going up the stack and eventually full recovery of hydrogen. The overall process includes the steps:
1) supplying a first stream of a feed gas containing hydrogen and nitrogen in a MOL ratio of about 2/1, and also containing methane and argon,
2) cryogenically separating the feed into the following:
a) a second stream of a synthesis gas containing hydrogen and nitrogen in a MOL ratio of about 3/1,
b) waste gas containing principally nitrogen, and also containing substantially all of the methane supplied in the first stream,
3) and splitting the waste gas into:
c) a third stream of nitrogen rich gas
d) a fourth stream of methane rich gas, useful as a fuel or as a feed to a subsequent process.
In that overall process, the second, third and fourth streams are typically delivered as product streams; and the second product stream of synthesis gas may be delivered to an ammonia synthesis process. Also, the nitrogen rich third stream may be recycled to the process air compressor.
Another object is to provide a nitrogen-methane separator for reception of waste gas derived from the initial separator in which synthesis gas was stripped from such waste gas. In this regard, the waste gas to be delivered to the nitrogen methane separator is typically passed through a cryogenic cold box for further cooling.
A further object is to pass the above described second, third and fourth streams through the cold box, for further cooling. Refrigeration for the cold box maybe produced by operation of an expander turbine.
Yet another object is to provide refrigeration by expansion of the waste gas over a Joule Thompson valve, prior to passage of the waste gas through a heat exchanger in which cooling of synthesis gas is effected, the waste gas then passing to the cold box.
An additional object is to re-compress the waste gas prior to the splitting step. Such re-compressed waste gas is then typically passed through the cold box, and the second, third and fourth streams are also passed through the cold box.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: